Internal combustion engine systems typically include exhaust gas after-treatment systems that receive and treat exhaust gas from an internal combustion engine. Exhaust gas after-treatment systems can include one or more exhaust gas treating elements, such as diesel oxidation catalysts, particulate matter filters, selective catalytic reduction systems, NOx adsorption catalysts, and ammonia oxidation catalysts. Commonly, the operation of such exhaust gas treatment elements depends on the temperature of the exhaust gas passing through the exhaust gas after-treatment system, and, more particularly, through the respective exhaust gas treating elements. Accordingly, exhaust gas after-treatment systems include temperature sensing devices, such as thermistors, to sense the temperature of the exhaust gas at one or more locations within the exhaust gas after-treatment systems.
For reading temperatures sensed by a thermistor, the thermistor is typically coupled with a voltage divider circuit that has a single interface or pull-up resistor with a constant resistance. Conventionally, the resistance value of the interface resistor is matched with the thermistor to provide accurate temperature readings for temperatures within a predefined temperature range. The predefined temperature range generally is associated with a predetermined operating range of the engine. For some diesel engines, the predefined temperature range can be between 200° C. and 650° C.
Problems associated with reading exhaust temperatures and diagnosing thermistor conditions may arise for exhaust gas temperatures that exceed the predefined temperature range or when out-of-range events occur. Although thermistors are capable of sensing exhaust temperatures outside of the predetermined temperature range, such as between 650° C. and 1,000° C., and between −40° C. and 200° C., thermistor interface circuits with a single pull-up resistor fail to provide accurate temperature readings and diagnostics within such extreme temperature ranges. For example, a thermistor interface circuit with a single pull-up resistor cannot discern between cold ambient conditions and an out-of-range condition, which could be the result of a defective thermistor.
To alleviate such shortcomings associated with thermistor interface circuits that have a single pull-up resistor, diagnostic testing of the thermistor typically is conducted during only those engine operating conditions when the exhaust gas is expected to be running within the predetermined temperature range. However, this approach introduces additional limitations, such as the unavailability of accurate diagnostic testing of the thermistor during certain operating conditions of the engine. For example, in cold weather environments, the temperature of exhaust gas may remain below the predetermined temperature range for prolonged periods of time. On Board Diagnostics (OBD) and other regulations may require that all temperature sensors work accurately, or defective sensors are quickly detected, through the entire potential operating range of an engine (e.g., exhaust temperatures between −40° C. and 1,000° C.). Accordingly, proper diagnosis of a defective thermistor may be unacceptably delayed when using a thermistor interface circuit with a single pull-up resistor.